More specifically, each blade extends lengthwise substantially radially, is rotated, in use, by the hub in a drive plane perpendicular to the axis of the drive shaft, and is movable in all planes with respect to the hub to manoeuvre the helicopter.
Established helicopter design criteria require that, for each operating condition of the helicopter, the work frequencies of the rotor and blades fall within a different range from that of the natural frequencies of the other component parts of the helicopter.
If such is not the case, the helicopter may be subjected to particularly severe, even destructive, vibration.
In helicopter design, appropriate setting of said natural frequencies is therefore essential; for which purpose, vibration damping devices are known to be interposed between the hub and each blade, either inwards or outwards of the blades.
Location of the damping device outwards of the blades produces additional aerodynamic drag which impairs performance and consumption of the helicopter.
To reduce such drag, the damping devices must be made as small as possible. By so doing, however, the loads acting on the rotor may induce severe stress on, and even cause detachment of, the damping devices, thus resulting in malfunctioning of the rotor and possibly additional stress on the helicopter, making it difficult to manoeuvre.
Moreover, if detached, the damping device may hit the helicopter rotor, with obvious consequences in terms of safety.
Conversely, location of the damping device inwards of the blade poses problems caused by the damping device interacting with the portion of the blade connected to the hub, and by sliding movements between the contacting parts.
More specifically, such sliding movements may produce wear and localized heating of the contacting parts, thus reducing the working life of the parts and dissipating energy at the expense of the efficiency of the helicopter.
Location of the damping devices inwards of the blades also poses size problems, making it necessary at times to redesign the rotor hub.